1. Field of the Invention
The present invention relates to a backup system against line defects and, in particular, relates to a backup apparatus against line defects and to a backup method against line defects in a system in which data are transmitted and received over an analog dedicated line in a normal state and over an ISDN line upon occurrence of a line defect.
2. Description of the Related Art
Data communication networks are becoming necessary infrastructures for companies' activities. Thus, if a network goes down, company activities are seriously affected. Therefore, highly reliable, low cost networks are strongly desired. To improve reliability, backup systems for switching from a dedicated line to another line upon an occurrence of a line defect have been used.
FIG. 1 (PRIOR ART) is a block diagram of a conventional backup system, comprising a host unit 1, a modem 2 on the host unit side, a dial backup unit (DBU) 3 on the host unit side, an analog dedicated line 4, a dial backup unit (DBU) 5 on a terminal unit side, a modem 6 on the terminal unit side, a terminal unit 7, and a public telephone line 9 for use in backing up data communications upon occurrence of a line defect.
The modems 2 and 6 are provided with a modulating portion, a demodulating portion, and a defect detecting portion. The modulating portion is for receiving a digital signal from the host unit 1 or the terminal unit 7, converting the digital signal into an analog signal in a voice band, and transmitting the analog signal to the line. The demodulating portion is for receiving an analog signal in a voice band from a line, converting the analog signal in the voice band into a digital signal, and transmitting the digital signal to the host unit 1 or the terminal unit 7. The defect detecting portion is for determining whether or not a received signal contains a carrier (carrier down) and whether or not the signal quality has deteriorated, indicating occurrence of a line defect.
If no line defect occurs, data communication is made between the host unit 1 and the terminal unit 7 over the analog dedicated line 4. However, if a line defect is detected, under the control of the dial backup units 3 and 5, data communication is made between the host unit 1 and the terminal unit 7 over the public telephone line 9. When the defect in the analog dedicated line restores from the defect, data communication is made over the analog dedicated line once again.
The amount of data transmitted over an analog line is theoretically up to 25 kbps. However, the actual amount of data available is 19.2 kbps at most. In addition, to accomplish a high speed communication of 19.2 kbps, the lines used should be of high quality (small deterioration factors). The conventional public telephone lines cannot satisfactorily support such high speed communication. Consequently, if data communication is backed up, the transmission speed decreases.
In addition, as shown in FIG. 2(A) (PRIOR ART), the conventional backup system only applies to a communication system with a point-to-point connecting structure. Thus, as shown in FIG. 2(B) (PRIOR ART), in a communication system with a multi-point connecting structure where data are transmitted from the host unit 1 over the dedicated line 4 and then branched to a plurality of terminal units, data communication cannot be backed up. In the multi-point connecting structure, until the dedicated line 4 restores from a defect, the system continues to be down.